Lactic acid production, separation and/or recovery process

ABSTRACT

A process for the production of lactic acid and for the separation and/or recovery of lactic acid from a lactate feed solution. A lactate feed solution preferably obtained from a fermentation broth is combined with and extracted by a water immiscible trialkyl amine in the presence of carbon dioxide. Lactic acid is recovered from the resulting organic phase. Recovered carbonate or bicarbonate from the aqueous phase is preferably recycled to the fermentor and regenerated extractant is preferably recycled for use in the extraction.

The present application is a continuation of application Ser. No.08/587,216 filed Jan. 16, 1996. Application Ser. No. 08/587,216 is acontinuation of application Ser. No. 08/207,773 filed Mar. 8, 1994, andwhich issued as U.S. Pat. No. 5,510,526 on Apr. 23, 1996. ApplicationSer. No. 08/207,773 is a continuation-in-part of application Ser. No.08/084,810 filed Jun. 29, 1993, now abandoned. The complete disclosuresof application Ser. Nos. 08/084,810; 08/207,773; and 08/587,216 areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the production, separationand/or recovery of lactic acid and more particularly to the production,separation and recovery of lactic acid via a fermentation process andthe separation and/or recovery of lactic acid from a lactate feedsolution such as is obtained from a fermentation broth or other sources.

2. Description of the Prior Art

Lactic acid has long been used as a food additive and in variouschemical and pharmaceutical applications. More recently, lactic acid hasbeen used in the making of biodegradable polymers both as a replacementfor present plastic materials as well as various new uses wherebiodegradability is needed or desired. Accordingly, there is an everincreasing demand for lactic acid. The present invention aims at meetingthis demand by providing an efficient and environmentally friendlyprocess for producing lactic acid which avoids the consumption of basesand acids and substantially reduces, if not eliminates, the formation ofwaste or byproduct salts.

Production of lactic acid is commonly carried out by fermentation of astrain of the bacterial genus Lactobacillus and more particularly by thespecies Lactobacillus delbrueckii or Lactobacillus acidophilus asexamples. In general, the production of lactic acid by fermentation in afermentation broth is well known in the art. The fermentation substrateconsists of carbohydrates together with suitable mineral andproteinaceous nutrients. Because the lactic acid producingmicroorganisms are inhibited in a strongly acidic environment, the pH ofthe fermentation broth must be kept above 4.5, and preferably within therange of about 5.0 to 7.0, more preferably within the range of about 5.5to 6.5, and most preferably within the range of about 6.0 to 6.5. Tomaintain this pH level, suitable water-soluble basic substances oragents that are non-toxic to the acid producing microorganism, such asalkali metal hydroxides, carbonates or bicarbonates or alkaline earthmetal hydroxides or carbonates, are commonly added to the fermentationbroth to neutralize the acid being produced. This results in theformation of a lactate solution rather than the desired lactic acidproduct. Such lactate solution contains the lactate anion and thecorresponding cation of the substance used to neutralize thefermentation broth.

Various methods have been proposed for the recovery of lactic acid froma fermentation broth. Where the fermentation is carried out in thepresence of calcium carbonate, it is possible to recover the lactic acidby acidification with sulfuric acid. This results in the precipitationof calcium sulfate, while free lactic acid remains in the mother liquor.If desired, the mother liquor may be concentrated to up to about 90% byweight lactic acid. Subsequently, lactic acid may be extracted from themother liquor with a suitable organic extractant to yield an extractwhich is back-extracted with water or the acid may be adsorbed on asuitable adsorbent and later desorbed. The resulting aqueous lactic addsolution may then be concentrated. This method has the disadvantage thatit irreversibly consumes calcium carbonate and sulfuric acid and leaves,as waste, large quantities of calcium sulfate which give rise todisposal problems.

U.S. Pat. No. 5,132,456 (King et al.) describes a process for recoveringcarboxylic add from a carboxylic acid-containing aqueous feed streamhaving a pH close to or above the pK_(a) level of the acid. Inaccordance with that process the recovery involves what may be describedas a cascade type acid withdrawal operation in which the basicity of theextractant is increased stepwise. In a first stage of the process, thefeed stream is contacted with an adsorbent such as a strongly basicextractant or a solid anion exchanger. In a second stage the acid-loadedadsorbent is contacted with an aqueous solution of ammonia or a lowmolecular weight trialkyl amine having a stronger affinity to thecarboxylic acid that is being recovered than the adsorber used in thefirst stage. In this way an aqueous solution of a water-solublecarboxylic acid ammonium salt is formed. This is then subjected to heattreatment, whereby the salt is decomposed to yield back the trialkylamine or ammonia and free carboxylic acid. Applying this process tolactic acid involves the formation of salts of lactic acid with strongbases having a pK_(a) value of about 9-11. Thus, the decomposition ofthese salts into free lactic acid is energy intensive. Examples 12-14 ofthe patent mention the use of Alamine 336 (tricaprylylamine) for theextraction of, among others, lactic acid from an aqueous solution, butno yields are mentioned. By the extraction of even small quantities oflactic acid from a fermentation broth the pH of the broth rises rapidlyto above 7. As shown in FIGS. 3 and 4 of this patent, the uptake ofcarboxylic acids from aqueous solutions drops rapidly with an increaseof the pH. It is, therefore, inherent in these examples that the lacticacid uptake, if any, is negligible. It is further noted that upon heattreatment and concentration of an ammonium lactate, crystalline lacticacid does not precipitate and instead the viscosity of the solutionsincreases steadily as a result of self-association of the acid. It isthus evident that the process of U.S. Pat. No. 5,132,456 is unsuitablefor the recovery of lactic acid from a fermentation broth.

U.S. Pat. Nos. 4,444,881 and 4,405,717 (Urbas) describe a process forthe recovery of an organic acid from a diluted aqueous solution of itscalcium salt by adding a water-soluble trialkyl amine carbonate to thesolution to form on the one hand a water soluble trialkyl ammonium saltof the acid, which salt remains in solution, and on the other handcalcium carbonate which precipitates. After removal of the calciumcarbonate the remaining mother liquor is heated for the separaterecovery of the amine and the product acid. The water-soluble trialkylamines employed in accordance with these patents are strongly basic.Accordingly, the decomposition of the trialkylammonium salts into freeacids is energy intensive.

U.S. Pat. No. 4,282,323 (Yates) describes a process for obtaining lowercarboxylic acids from a salt solution of such carboxylic acid asobtained from fermentation. The process appears to be applicable to arestricted number of lower aliphatic and aromatic monocarboxylic acidsand is specifically described only in relation to acetic acid. Inaccordance with that process, the aqueous solution of a carboxylic acidsalt is contacted in the presence of a liquid polar organic solventserving as extractant, with pressurized carbon dioxide, to convert atleast part of the salt to the corresponding free acid which is taken upby the organic phase from where it is subsequently recovered. It isinherent in the use of a polar organic extractant that the bulk of thecarboxylic acid remains in the neutral to basic aqueous phase, andindeed the recovery rates reported in U.S. Pat. No. 4,282,323 are low,ranging between 4.8% and 18% of the acid initially present.

U.S. Pat. No. 4,275,234 (Baniel) is directed to a method of recoveringvarious acids in their free form from aqueous solutions. Thus, theprocess of Baniel is not applicable to a lactate solution of the typecommonly obtained from a fermentation process or from other sources. Theessence of the Baniel U.S. Pat. No. 4,275,234 is the discovery thatefficient back-extraction can be achieved by performing theback-extraction at a temperature higher than that of the primaryextraction.

R. Bar and J. L. Geiner, Biotechnology Progress 3, 109 (1987) studiedthe feasibility of extracting lactic acid from aqueous solution by meansof a long-chain trialkyl amine of low basicity, such as tridodecylamine,using various tridodecylamine solutions in n-dodecanol. It was foundthat extraction of lactic acid by extraction with a long-chain trialkylamine such as tridodecylamine was effective only at a pH that is lowerthan the pK_(a) of lactic acid, the latter of which is 3.86. At such alow pH, however, the lactic acid fermenting microorganism such as, forexample, Lactobacillus delbrueckii or Lactobacillus acidophilus isseverely inhibited.

To sum up, the prior art teaches that when using amines for the recoveryof lactic acid from a lactate solution such as that obtained from afermentation broth, either by way of extraction or by way of ammonuimsalt formation, only strongly basic water soluble amines can be used,while water immiscible amines of weak basicity, such as long-chaintrialkyl amines, are unsuitable. Accordingly, a need exists for a lacticacid production process which is energy efficient, is environmentallyfriendly, consumes no acids or bases and which minimizes, if noteliminates, generation of waste salts or emissions.

SUMMARY OF THE INVENTION

In accordance with the present invention, and contrary to the teachingsof the prior art, it has surprisingly been found that it is possible toseparate and recover lactic acid from a lactate solution at a pH in therange of 4 to 14 in a nearly quantitative fashion by a process meetingthe above objectives. More specifically, the lactic acid separation andrecovery process includes an extraction (hereinafter sometimes referredto as the primary or forward extraction) in the presence of a waterimmiscible, long-chain trialkyl amine and carbon dioxide. The lactatesolution may be obtained from a fermentation broth or from hydrolyzedpolylactide via polylactide recycling or recovery, among possibleothers.

Preferably the invention provides a process for the separation and/orrecovery of lactic acid from a lactate solution formed by fermentationin the presence of a basic substance selected from the group of alkalimetal, alkaline earth metal or ammonium hydroxides, carbonates orbicarbonates. The process steps comprise obtaining a lactate feedsolution from a fermentation broth or another source and combining suchfeed solution with an extractant comprising a trialkyl amine in thepresence of carbon dioxide, where the trialkyl amine is water immiscibleand has a total of at least 18 carbon atoms. The term "combining" asused herein is intended to mean a mixing or contacting of the lactatesolution and the amine so that extraction can occur. Preferably thelactate feed solution is formed by filtering a fermentation broth toremove biomass and other solids and the combining of the lactatesolution and extractant occurs in the presence of carbon dioxide at apartial pressure of at least about 50 psig.

The above extraction in accordance with the present invention results inthe formation of a lactic acid rich organic phase and an aqueous oraqueous-slurry phase. Each of these two phases, in accordance withpreferred further aspects of the invention, is processed to recoverlactic acid from the organic phase and carbonate or bicarbonate from theaqueous phase. Preferably, the recovered carbonate or bicarbonate isrecycled to the fermentor. The organic phase from which the lactic acidhas been recovered is recycled for use in the primary extraction. Thisresults in a process in which the consumption of acids and bases isavoided and in which the generation of waste salts and other by-productsis substantially reduced, if not eliminated.

In a preferred process of the present invention, a countercurrentliquid--liquid extractor or extraction unit is used. During steady stateoperation, the lactate feed solution and extractant are loaded into theextractor and operated in the presence of pressurized carbon dioxide.The optimum operational pressure or pressure range may be establishedfor each particular case. The particular pressure is not criticalprovided sufficient carbon dioxide is present for the primary extractionto occur. Preferably the partial pressure of carbon dioxide ismaintained at 50 psig or greater. Upon leaving the extractor, theorganic phase may be subjected to decompression. This results in arelease of the pressurized carbon dioxide which can, if desired, berecovered for reuse in the process.

The long-chain trialkyl amines useful in accordance with the presentinvention are those in which the amines and the amine lactate salts areimmiscible with water and have a total of at least 18 carbon atoms, andpreferably from 24 to 42 carbon atoms. Typical examples of such aminesare trihexylamine, trioctylamine, triisooctylamine, tricaprylylamine andtridodecylamine. As used herein, the term amine salt or amine lactatesalt refers to the species formed when lactic acid is extracted into theamine extractant phase, although the exact nature of this species is notknown.

The extraction process of the present invention may be performedbatchwise or continuously, but dramatically improved separation andultimate recovery can be achieved with a continuous process and inparticular a countercurrent extraction process.

Solvents of the trialkyl amines of the present process may also be used,if desired, as part of the extractant. These may be used for the purposeof diluting certain relatively viscous trialkyl amines, enhancing theextraction, and/or stabilizing and maintaining the organic phase in asingle phase substantially immiscible with water. Any compatible organicsolvent capable of dissolving the amine and the amine lactate salt issuitable provided it is also inert to chemical reaction both with thelong-chain trialkyl amines utilized and to the amine lactate salt andlactic acid. As used herein, the term "compatible" means miscible with,soluble in and chemically inert. The usefulness of solvents for thesepurposes is well known in the art. Specific examples, however, includeliquid hydrocarbons such as kerosene or mineral oils, alkanols such asisopropanol, n-butanol and n-octanol and various ketones such asmethyl-isobutyl ketone (MiBK) and nonanone, among others. If desired,two or more different solvents may be used, e.g. a hydrocarbon and analkanol.

The organic phase resulting from the primary or forward extraction issubjected to a separation process such as further extraction,vaporization or the like to recover the lactic acid. Preferably, theorganic phase is subjected to back-extraction with water to recover thelactic acid in an aqueous phase. Where the initial extracting mediumalso contains an alkanol or ketone as a solvent, the back-extraction maybe preceded by removal of the solvent through azeotropic steamdistillation or other techniques. That portion of the organic phaseremaining after separation of the lactic acid and, where applicable, theseparately recovered alkanol or ketone, may be recycled for use in theprimary extraction. The aqueous lactic acid solution resulting from theback-extraction is removed as product and may be concentrated, ifdesired.

In a preferred embodiment of the process, carbonate or bicarbonate ispresent in the aqueous phase either in solution or as a solidsuspension, predominantly in the form of an alkali metal, alkaline earthmetal or ammonium carbonate or bicarbonate, depending on the cationpresent in the lactate solution. This aqueous phase is preferably asuspension of sodium bicarbonate crystals and is subjected tosolid-liquid separation followed by conversion of the bicarbonate intosodium carbonate by heat treatment or other techniques known in the art.Carbon dioxide liberated during this conversion may be trapped andrecycled for use in the primary extraction. The solid-liquid separationalso yields an aqueous raffinate substantially depleted of lactate whichis withdrawn and may be used as a constituent of animal feed.

Accordingly, it is an object of the present invention to provide aprocess for separating and/or recovering lactic acid from a lactatesolution which does not involve consumption of acids and bases and whichavoids generation of waste salts.

A further object of the present invention is to provide a process forseparating and/or recovering lactic acid from a lactate solution whichrequires minimal energy use and results in minimal, if any, plantemissions.

A still further object of the present invention is to provide a processfor producing lactic acid from a fermentation process which usesenvironmentally friendly materials and in which substantially all byproducts of the production process are recycled.

Another object of the present invention is to provide a process forseparating and/or recovering lactic acid from a lactate feed solutionutilizing a long-chain trialkyl amine in the presence of carbon dioxide.

These and other objects of the present invention will become apparentwith reference to the drawing, the description of the preferredembodiment and process, the examples and the appended claims.

DESCRIPTION OF THE DRAWING

The single FIGURE of the drawing is a block diagram representing thepreferred embodiment of the process according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT AND PROCESS

With reference to the drawing, lactic acid fermentation is carried outin a fermentor 10 in which carbohydrates are fermented and converted tolactic acid by the bacterial genus Lactobacillus and more specificallyby the microorganism Lactobacillus acidophilus. Because many organismswhich are attractive in such a fermentation process cannot tolerateacidic conditions with a pH lower than about 3.8, the adds formed bythis process must be at least partly neutralized to maintain the pHabove such level and more preferably above a pH of 4.5 to allow thefermentation to continue. In accordance with the preferred process, aneutralizing agent such as the alkali metal, alkaline earth metal orammonium hydroxides, carbonates or bicarbonates are used for thispurpose In the preferred process, sodium carbonate (Na₂ CO₃) is added tothe fermentor 10 for this purpose, either via the recycle 12 asdiscussed below or along the path 13. Preferably, sufficient sodiumcarbonate or other alkaline substance is provided to the fermentor 10 tomaintain the pH of the fermentation broth at a pH above 5.0 andpreferably in the range of about 5.0 to 7.0, more preferably in therange of about 5.5 to 6.5 and most preferably in the range of about 6.0to 6.5. Other ingredients may also be used in the fermentation processwhich is well known in the art.

In the fermentor 10, the carbohydrate is converted to lactic acid whichimmediately is converted to a lactate form in the presence of theneutralizing agent. In the preferred process using sodium carbonate,sodium lactate NaCH₃ CH(OH)COO! is formed. A portion of the fermentationbroth or liquor is continuously or intermittently withdrawn from thefermentor 10 via the path 14 and exposed to a filtration andconcentration unit 15. The unit 15 functions to physically remove, viafiltration or ultrafiltration, biomass and other solids which can berecycled to the fermentor 10, if desired. The filtrate comprises anaqueous lactate solution which contains the lactate salt comprised ofthe lactate anion together with the cation of the neutralizing agent. Inthe preferred process, the filtrate is comprised principally of sodiumlactate. This solution, which commonly comprises between about 0.25% and50% by weight of sodium lactate, may be concentrated by waterevaporation or other techniques to improve the overall lactic acidproduction efficiency. In the preferred process, the filtered lactatesolution is concentrated by water evaporation to about 40% to 70% byweight sodium lactate; however, such concentration is optional.

The sodium lactate solution exiting from the filtration andconcentration unit 15 comprises a lactate feed solution which is fedinto an extraction unit 18 along the path 16. The unit 18 is part of anextraction system which also includes the extractant regeneration unit22, the organic phase stream 21 and the extractant recycle stream 24.Within the unit 18, the lactate feed solution is combined with anextractant comprised of at least one water immiscible trialkyl amine inthe presence of carbon dioxide, where the amine has a total of at least18 carbon atoms. Within the unit 18, two separate phases are formed: anorganic phase containing the extractant and extracted lactic acid and anaqueous or aqueous-slurry phase containing the carbonate or bicarbonatesalt of the cation of the neutralizing agent. In the preferred process,the aqueous phase contains sodium carbonate or bicarbonate. The unit 18may comprise any one of a variety of single or multi-stage pressureextraction units. In the preferred process, the unit 18 is a multi-stagecountercurrent extraction unit.

In the preferred process, the extraction system is initially chargedwith the trialkyl amine. The amine may be introduced by directly addingit to the unit 18 or by adding it to the recycle stream 24 through theamine make-up stream 17. During steady state operation, little if anyadditional trialkyl amine will be needed. To the extent it is, however,it can be added to the recycle stream 24 via the make-up stream 17.

Carbon dioxide may be added directly to the unit 18 under pressure viathe path 19a, to the organic recycle stream 24 under pressure via thepath 19c, or to the aqueous lactate stream 16 under pressure via thepath 19b. In the preferred process as illustrated, the organic recyclestream 24 is preloaded with carbon dioxide by adding carbon dioxideunder pressure via the stream 19c prior to the unit 18. In any case, thecarbon dioxide within the unit 18 should preferably be maintained at apartial pressure of at least about 50 psig, more preferably at a partialpressure of at least 75 psig and most preferably between about 150-300psig.

Although it is believed that some extraction of lactic acid from alactate solution is possible with any water immiscible trialkyl amine inthe presence of carbon dioxide, the particular degree of extraction willvary with the amine utilized and the carbon dioxide pressure. The degreeof extraction can also be enhanced or otherwise affected by varioussolvents as described below and as known in the art. The degree ofextraction will generally be dependent on the partition coefficient andthe number of stages used in the extraction process. As used herein, thepartition coefficient is the mass concentration of lactic anion in theorganic phase divided by the mass concentration of lactate expressed aslactic acid equivalent in the aqueous phase. Usually, for a particularsystem, the partition coefficient, within limits, will vary directlywith the carbon dioxide pressure. As the partition coefficientincreases, the number of stages needed to achieve a particular degree ofextraction will decrease. Carbon dioxide and amine composition shouldpreferably be sufficiently high to avoid excessive extractant phasenecessary to extract the add.

The trialkyl amines which are useful in the process of the presentinvention are those which are water immiscible and relatively weak.Specifically, these are the trialkyl amines having a total of at least18 carbon atoms and preferably about 24 to 42 carbon atoms. Thepractical lower limit of the number of carbon atoms is limited by theincreasing water solubility of the smaller trialkyl amines or theirsalts. The water immiscibility of the trialkyl amines with 18 or morecarbon atoms is well known in the art. The practical upper limit of thenumber of amine carbon atoms is determined by the molar concentration ofamine obtainable in the organic phase. Specifically, the extractionability of the trialkyl amines is dependent on the molar concentrationof the amine component. Thus, as the molecular weight of the amineincreases, the molar concentration of the amine component (or a pureamine solution) will decrease. The trialkyl amine should also besufficiently strong to extract the lactic acid from the aqueous lactatefeed, but sufficiently weak to enable water to back extract the lacticacid from the organic phase. Typical examples of such amines which meetthe above requirements, are readily available and are useful in theprocess of the present invention are one or more of trihexylamine,trioctylamine, triisooctylamine, tricaprylylamine, tridodecylamine andmixtures thereof.

The particular ratios of lactate feed solution and trialkyl amine phasewhich are fed to the unit 18 along the paths 16 and 24, respectively,will depend on a variety of factors including the concentration of thesodium lactate and the concentration of the amine. Preferably, theintroduction of these materials should be such as to result in asubstantially complete extraction of lactic acid from the lactatesolution with the number of stages utilized. More preferably, the feedratio of amine phase to lactate solution should be about 40:1 to 1:2 andmost preferably about 15:1 to 1:1 by weight.

The trialkyl amine provided to the unit 18 may be introduced in asubstantially pure or a diluted form. Because many of the aminesapplicable to the present process and their salts are relativelyviscous, it is preferable to introduce such amines with a solvent. Ingeneral, any composition which is miscible with the subject amines andtheir salts within the range of compositions used and which isreactively inert relative to the system components may be used in thepresent process. These solvents may be used to control viscosity,enhance extraction or stabilize the organic phase in a manner generallyknown in the art. Typical examples of solvents which can be used in thepresent process include liquid hydrocarbons such as kerosene or mineraloil, alkanols such as isopropanol, n-butanol and n-octanol and variousketones such as methyl-isobutyl ketone (MiBK) and nonanone, amongothers. The extractant used in the process of the present invention maycomprise 100% of the trialkyl amine. A more preferred extractant,however, comprises up to about 70% by weight of a solvent or shouldcomprise about 30%-95% by weight of the amine and about 5%-70% by weightof the solvent.

Following extraction within the unit 18, a lactic acid-rich organicphase comprised of lactic acid and the extractant is withdrawn along thepath 21 and an aqueous phase or slurry comprised principally ofcarbonate and/or bicarbonate is withdrawn along the path 20. As used inthe description of the preferred embodiment, the term carbonate orbicarbonate refers to the carbonate or bicarbonate salt of the cation ofthe substance used to neutralize the fermentation. Within the aqueousphase or slurry of the preferred embodiment, the predominant carbonateor bicarbonate is sodium bicarbonate which exists principally as sodiumbicarbonate crystals. These are separated from the aqueous raffinate inthe solid-liquid separation unit 25. The unit 25 can comprise variousfiltration, centrifugation or other solid-liquid separation means knownin the art. Preferably, however, the sodium bicarbonate crystals areseparated by filtration. The aqueous filtrate which in the preferredprocess is substantially free of lactate may be removed as a componentof animal feed or as waste along the path 26. It is also possible, ifdesired, to recycle all or part of the filtrate back into the systemthrough the streams 11, 14 or 16.

The separated sodium bicarbonate is then directed along the path 29 to aconversion and purification unit 30 for conversion of the sodiumbicarbonate to sodium carbonate. Means are known in the art foraccomplishing this conversion. In the preferred process, however, thesodium bicarbonate crystals are decomposed in boiling water to producecarbon dioxide and dissolved sodium carbonate. The solution of sodiumcarbonate is then purified by active carbon treatment and recycled alongthe path 12 as an alkaline or neutralization component in thefermentation process. The released carbon dioxide can also be reused, ifdesired. Since the preferred process utilizes sodium carbonate as theneutralizing component in the fermentation process, the aqueous phaseafter fermentation (stream 14) is comprised of sodium lactate. It iscontemplated that other alkali metals, alkaline earth metals or ammoniumhydroxides, carbonates or bicarbonates may also be used as theneutralizing agent, in which event the cations in the aqueous phasewould be altered accordingly.

The lactic acid-rich organic phase is withdrawn from the unit 18 alongthe path 21. In the preferred process, the organic phase is decompressedin the flash unit 32 as it leaves the unit 18. This results in therelease of a majority of the dissolved carbon dioxide via the stream 33which may be recycled to streams 19a, 19b or 19c, if desired. Thisorganic phase is made up principally of lactic acid and the extractant.Lactic acid is separated or recovered from this phase in the extractionunit 22, leaving a lactic acid-lean or depleted organic phase which ispreferably recycled back to the extraction unit 18 along the path 24 inthe form of regenerated extractant. As described above, carbon dioxidemay also added to the recycle stream 24 via the path 19c to load theamine prior to the unit 18. The lactic acid solution is removed from theunit 22 as product via the stream 28.

In the preferred process, the unit 22 is a liquid/liquid extraction unitwithin which the lactic acid-rich organic phase is back extracted withwater introduced along the path 23. Because of the relatively weak aminebeing used in the primary extraction process and because the amine iswater immiscible, the water is able to extract the lactic acid from theamine to form an aqueous solution of lactic acid of acceptableconcentration.

In the case where the trialkyl amine is diluted with an appropriatesolvent, such solvent becomes a part of the organic phase withdrawn fromthe unit 18 along the path 21. Some solvents, such as alkanols andketones, modify and enhance the lactic add uptake into the organicphase. It is preferable to remove such solvents prior to the backextraction with water in the unit 22. This separation of the solventfrom the lactic add-rich organic phase can be accomplished by variousseparation techniques known in the art. Preferably, when possible, theseparation is by azeotropic steam distillation within the unit 27. Theremoved solvent from the separation unit 27 may then be recycled alongthe path 31 for regeneration of the extractant and use in the primaryextraction, if desired.

The lactic acid can also be separated or recovered from the organicphase by vaporization or distillation of the lactic acid. Removal bydistillation should preferably be performed at reduced pressure andelevated temperature conditions. Most preferably, the separation shouldbe accomplished at pressures of from about 0.2 to 100 mm Hg and attemperatures from about 80° C. to about 240° C. If this distillationoption is employed, the trialkyl amine should have a total of at least24 carbon atoms, or be sufficiently nonvolatile to allow lactic acidfractionation from the amine by vacuum distillation.

The vaporization conditions will also remove alkanols or ketones, ifpresent, as well as other solvent components more volatile than thetrialkyl amine. These can be separately recovered and may be returned tothe depleted extractant before it is cycled back to the extraction step.The vapor of lactic add thus formed may also be directly fed, ifdesired, to an esterification process for reaction and furtherpurification.

With the above process, lactic acid can be separated and/or removed froma lactate fermentation broth. Under optimal conditions, such separationand/or recovery can approach total recovery of the lactic acid, greaterthan 95% of that produced by fermentation. Of equal or greaterimportance is the ability of this recovery to be accomplished with thegeneration of minimal, if any, waste salt and under circumstances wheresubstantially all of the extraction, conversion and other componentsused in the process can be recycled for reuse within the process. Stillfurther, the process is significantly less energy intensive thancompeting processes and results in minimal, if any, plant emissions.

The preferred process has been described with respect to producinglactic acid from a lactate solution formed via a fermentation process.The present process is, however, applicable to the separation and/orrecovery of lactic acid from a lactate solution regardless of itsorigin. For example, polylactide is a biodegradable polymer producedfrom lactic acid. Polylactide can be recycled by hydrolysis of thepolymer to yield a lactate salt. The present process can then be used torecover lactic acid from the lactate salt for reuse in formation of thepolylactide polymer.

Further details of the present process are shown and described in thefollowing specific examples.

EXAMPLE 1

A lactate fermentation broth containing 10% by weight of NaCH₃ CH(OH)COO(sodium lactate) was withdrawn from a fermentor in which purecarbohydrates were fermented by Lactobacillus delbrueckii in thepresence of sodium carbonate in order to maintain a pH of 5.5, all asknown in the art. The biomass and other solids were removed from thefermentation broth by filtration through ultrafiltration membranes andthen concentrated by water evaporation to 50% by weight sodium lactate.

150 g/min of this sodium lactate solution were fed to a 5-stagemixer-settler battery counter-currently to 1050 g/min of regeneratedextractant comprising 48% by weight tricaprylylamine (Alamine 336™produced by Henkel), 20% by weight n-butanol and 32% by weightaromatic-free kerosene. The extraction system was kept at ambienttemperature and a 240 psig CO₂ atmosphere was maintained therein. Sodiumbicarbonate crystals formed in the aqueous phase as of the secondmixer-settler.

The aqueous phase was withdrawn and sodium bicarbonate was filtered offfrom the aqueous raffinate which was practically free of lactate values.The sodium bicarbonate crystals were decomposed in boiling water to CO₂and to dissolved sodium carbonate. This solution was purified by activecarbon treatment to a form suitable for use as a base in thefermentation.

The organic phase withdrawn from the last stage of the mixer-settlerbattery contained 0.65 mole lactic acid per kg. CO₂ was allowed toescape from the organic phase, following which the butanol was separatedand removed by azeotropic steam distillation. The remaining organicphase was back-extracted with hot water to form a lactic acid-depletedorganic phase and an aqueous solution of the lactic acid. The separatedbutanol was reintroduced into the back-extracted organic phase toregenerate the extractant while the aqueous phase was concentrated andfed to final purification. Removal efficiency of lactic acid from thelactate feed solution was about 95%.

EXAMPLE 2

Aqueous solutions of sodium lactate were equilibrated in a pressurevessel with various extractants under a 220 psig CO₂ atmosphere. Contacttemperature was 20° C. The initial pH of the aqueous phase andequilibrium data are summarized in the Table below.

                  TABLE    ______________________________________                 Initial                       Equilibrium data                   aqueous sodium lactate                                      lactic acid    Extractant composition                   pH      aqueous (wt %)                                      organic (wt %)    ______________________________________    n-butanol      5.5     62         1.05    80% TDA + 20% i-ProH                   5.5     29.2       10.6    80% TDA + 20% hexane                   5.5     39.8       2.9    67% TCA + 33% n-OctOH                   10.9    50.3       14.3    70% TCA + 30% n-BuOH                   10.9    50.3       15.0    48% TCA + 20% n-BuOH +                   10.7    46.9       9.3    32% kerosene    ______________________________________     TDA = tridodecylamine (Henkel)     TCA = tricaprylylamine     iPrOH = isopropanol     nOctOH = noctanol     nBuOH = nbutanol

EXAMPLE 3

An extractant mixture comprised of 80% by weight tridodecylamine(Alamine 304-1 produced by Henkel) and 20% by weight n-butanol, wascontacted with 30% by weight aqueous lactic acid (Purac) in sufficientquantity to produce a loading of 6.9% by weight lactic acid in theorganic phase. 230 g of this material was added to a stirred roundbottom flask connected to a distillation apparatus, condenser, andcontrolled vacuum system. The solution was heated to 219° C. at apressure of 2 mm Hg. Initial condensate fractions included butanol andwater. A later fraction showed recovery of 97% by weight of the originallactic acid. The residual extractant contained 0.2% by weight lacticacid. The composition of the pooled fractions containing the acid was98.3% by weight aqueous lactic acid and 1.7% by weight of its oligomers.The depleted extractant was replenished with butanol and cycled back foranother extraction. Five such cycles were run on one batch withoutsignificant loss of extractant performance.

EXAMPLE 4

Various experiments were conducted in a Parr pressure reactor for thepurpose of showing the applicability of the process of the presentinvention to a broad range of CO₂ pressures, to a variety of solventsand to representative samples of trialkyl water immiscible amines with atotal carbon content of at least 18.

The apparatus comprised a Parr pressure reactor with four agitators, gasinlet and outlet ports, aqueous and organic sample ports and a pressuregage. The procedure involved adding the aqueous and the pre-mixedorganic solutions to the Parr reactor. Except for Experiment Nos. 7, 12,18 and 21 below in which the ratio of organic to aqueous was 1:3, theratio of organic to aqueous in all experiments was 1:1. The aqueoussolution was comprised of sodium lactate (NaLa), calcium lactate (CaLa)or potassium lactate (KLa). The NaLa solutions comprised about 20%-40%by weight of the lactate, the KLa solution comprised about 20% by weightof the lactate, while the CaLa solution comprised about 6% by weight ofthe lactate. Further, in Experiment Nos. 7-10, 12-18 and 20-24, 10% byweight sodium bicarbonate was added for the purpose of saturating thesolution. The organic solution comprised a trialkyl amine or a mixtureof a trialkyl amine and one or more solvents.

The Parr pressure reactor was then assembled and a slow flow of CO₂ wasintroduced for about 5 minutes to purge the air in the reactor. The CO₂pressure was then adjusted to the desired level. It should be noted thatthe pressures identified in the table below are gage pressures. Thus, aCO₂ level of O psig as indicated in Experiment Nos. 1 and 13 reflect aCO₂ partial pressure of 14.7 psi. The CO₂ pressure was maintained at theselected level, with a slow bleed of CO₂ (about 100 ml/min) bubblingthrough the contents, and the contents in the reactor were agitated fortwo hours. The CO₂ inlet and outlet ports were then sealed and agitationcontinued for 10 more minutes, at which time agitation was terminatedand the contents were allowed to settle for 30 minutes. Samples of boththe organic and aqueous phases were collected through the organic andaqueous sample ports, after which the above procedure repeated for adifferent CO₂ pressure. All experiments were run at 25° C.

All organic samples were analyzed with NaOH to a phenolphthaleinendpoint to determine concentration of lactic add in the organic phase.All aqueous samples were analyzed by HPLC to determine lactic acidequivalent in the aqueous phase. The partition coefficient (K) was thencalculated by dividing the concentration of lactic acid in the organicphase by the concentration of lactate, expressed as lactic acidequivalent, in the aqueous phase.

The table below reflects data from selected experiments conducted inaccordance with the above procedure in which the amines and solvents areidentified as follows. All percentages are by weight unless otherwisespecified.

    ______________________________________    Amines             Solvents    ______________________________________    A1 = trihexylamine CS1 = n-octanol    A2 = trioctylamine CS2 = n-butanol    A3 = triisooctylamine                       CS3 = nonanone    A4 = tricaprylylamine                       CS4 = Isopar K, Exxon    A5 = tridodecylamine    ______________________________________

    __________________________________________________________________________                          CO.sub.2 Press.                               K      Final Aq.    No.       Organic       Aqueous                          (psig)                               C(org)/C(aq.)                                      Wt % Lac.    __________________________________________________________________________    1  A4(48%), CS1(30%), CS4(22%)                     CaLa 0    0.068  2.21    2  A4(48%), CS1(30%), CS4(22%)                     CaLa 150  0.388  2.06     3.       A4(48%), CS1(30%), CS4(22%)                     CaLa 220  0.553  l.99    4  A4(48%), CS1(30%), CS4(22%)                     CaLa 300  0.622  1.93    5  A4(48%), CS1(30%), CS4(22%)                     KLa  75   0.112  18.8    6  A4(48%), CS1(30%), CS4(22%)                     KLa  500  0.198  17.5    7  A4(48%), CS1(30%), CS4(22%)                     NaLa (sat)                          75   0.047  19.9    8  A4(48%), CS1(40%), CS4(12%)                     NaLa (sat)                          150  0.093  38.3    9  A4(48%), CS1(40%), CS4(12%)                     NaLa (sat)                          200  0.155  38.0    10 A4(48%), CS1(40%), CS4(12%)                     NaLa (sat)                          300  0.199  37.5    11 A4(48%), CS2(35%), CS4(17%)                     NaLa 220  0.255  29.8    12 A1(33%), CS1(30%), CS4(37%)                     NaLa (sat)                          240  0.123  20.1    13 A2(43%), CS1(30%), CS4(27%)                     NaLa (sat)                          0    0.038  21.9    14 A2(43%), CS1(30%), CS4(27%)                     NaLa (sat)                          75   0.064  21.6    15 A2(43%), CS1(30%), CS4(27%)                     NaLa (sat)                          240  0.119  20.7    16 A4(48%), CS3(52%)                     NaLa (sat)                          240  0.069  21.6    17 A3(43%), CS1(30%), CS4(27%)                     NaLa (sat)                          75   0.038  20.3    18 A3(43%), CS1(30%), CS4(27%)                     NaLa (sat)                          240  0.097  20.0    19 A5(48%), CS2(20%), CS4(32%)                     NaLa 220  0.266  34.4    20 A4(100%)      NaLa (sat)                          75   0.020  21.4    21 A4(100%)      NaLa (sat)                          240  0.047  21.4    22 A4(43%), CS1(30%), CS4(27%)                     NaLa (sat)                          500  0.208  19.8    23 A4(48%), CS3(52%)                     NaLa (sat)                          500  0.102  21.3    24 A4(48%), CS4(52%)                     NaLa (sat)                          500  0.020  21.8    __________________________________________________________________________

We claim:
 1. A process for recovery of lactic acid; said processincluding steps of:(a) treating a lactic acid fermentation broth with amaterial to form an aqueous solution of lactate salt; (b) treating anaqueous solution of lactate salt with an agent to form:(i) a first saltselected from the group consisting of carbonate and bicarbonate salts;and, (ii) second lactate salt; (c) separating the first salt and thesecond lactate salt; (d) using at least a portion of the separated firstsalt from step (c) to treat a lactic acid fermentation broth accordingto step (a); (e) recovering lactic acid from the second lactate salt andforming an agent capable of treating lactate salt to form the secondlactate salt; (f) said process for recovery at least including stepsof:(i) forming a system including at least: an aqueous phase; and, anon-aqueous phase containing an amine lactate salt formed from awater-immiscible tertiary amine; (ii) separating the non-aqueous phasefrom the aqueous phase; and, (iii) recovering lactic acid from thenon-aqueous phase by extraction; and, (g) said process further includinga step of using at least a portion of the agent formed in step (e) totreat the lactate salt in step (b).
 2. A process according to claim 1wherein:(a) said step (f)(iii) of recovering comprises:(i) forming afurther two phase system by adding an aqueous phase to a separatednon-aqueous phase from step (f)(ii); (ii) generating lactic acid in theaqueous phase of the further two phase system; (iii) separating theaqueous phase containing lactic acid from step 2(a)(ii) from thenon-aqueous phase; and, (iv) isolating lactic acid from a separatedaqueous phase from step 2(a)(iii).
 3. A process according to claim 1wherein:(a) said step (1)(f)(iii) of recovering by extraction comprisesrecovering by extraction into water.
 4. A process according to claim 1wherein:(a) said step of treating a lactic acid fermentation broth withmaterial to form an aqueous solution of a lactate salt comprisestreating the fermentation broth with material selected from the groupconsisting of carbonate salts and bicarbonate salts.
 5. A processaccording to claim 4 wherein:(a) said step of forming the second lactatesalt comprises forming an amine lactate salt.
 6. A process according toclaim 5 wherein:(a) said step of forming an agent capable of treatinglactate salt to form the second lactate salt includes isolating tertiaryamine.
 7. A processing according to claim 1 wherein:(a) said step oftreating the fermentation broth comprises treating the fermentationbroth with an alkaline earth metal carbonate.
 8. A process according toclaim 7 wherein:(a) said step of treating an aqueous solution of lactatesalt with an agent to form a first salt comprises treating the aqueoussolution with an agent to form an alkaline earth metal carbonate.
 9. Aprocess according to claim 1 wherein:(a) said step of treating a lacticacid fermentation broth with material to form an aqueous solution oflactate salt comprises treating the lactic acid fermentation broth withalkaline earth metal salt; and, (b) said step of treating the aqueoussolution of lactate salt to form a first salt comprises treating to forma carbonate.
 10. A process according to claim 1 including:(a) a step offorming an alkali metal bicarbonate.
 11. A process according to claim 10wherein:(a) said step of forming an alkali metal bicarbonate comprisessaid step of treating an aqueous solution of lactate salt to form afirst salt and the second lactate salt.
 12. A process according to claim11 wherein:(a) said step of forming an alkali metal bicarbonatecomprises treating an aqueous solution of lactate salt with a tertiaryamine and CO₂ to form an organic phase soluble amine lactate salt.
 13. Aprocessing according to claim 1 wherein:(a) said step of treating anaqueous solution of lactate salt to form a first salt and a secondlactate salt comprises treating the aqueous solution of lactate saltformed in step 1(a).
 14. A process according to claim 1 wherein:(a) saidstep of treating lactic acid fermentation broth with a material to forman aqueous solution of lactate salt comprises treating fermentationbroth with an alkaline earth metal carbonate.
 15. A process according toclaim 14 wherein:(a) said step of treating an aqueous solution with anagent to form second lactate salt comprises forming an amine lactatesalt; and (b) said step of treating the aqueous solution of lactate saltto form second lactate salt includes treating with CO₂.
 16. A processaccording to claim 15 wherein:(a) said step of treating the aqueoussolution of lactate salt with an agent to form second lactate saltcomprises forming an amine lactate salt.
 17. A process for recovery oflactic acid; said process including steps of:(a) treating a lactic acidfermentation broth with a material to form an aqueous solution oflactate salt; (b) treating an aqueous solution of lactate salt with anagent to form:(i) a first salt selected from the group consisting ofcarbonate and bicarbonate salts; and (ii) second lactate salt; (c)separating the first salt and the second lactate salt; (d) using atleast a portion of the separated first salt from step (c) to treat alactic acid fermentation broth according to step (a); and, (e)recovering lactic acid from the second lactate salt and forming an agentcapable of treating lactate salt to form the second lactate salt; (f)said process for recovery at least including steps of:(i) forming asystem including at least: an aqueous phase; and, a non-aqueous phasecontaining an amine lactate salt formed from a water-immiscible tertiaryamine having at least 18 carbon atoms; and; (ii) separating thenon-aqueous phase from the aqueous phase; (g) said process furtherincluding a step of using at least a portion of the agent formed in step(e) to treat the lactate salt in step (b).
 18. A process according toclaim 17 wherein:(a) said step of treating a lactic acid fermentationbroth with material to form an aqueous solution of a lactate saltcomprises treating the fermentation broth with material selected fromthe group consisting of carbonate salts and bicarbonate salts.
 19. Aprocess according to claim 18 wherein:(a) said step of forming thesecond lactate salt comprises forming an amine lactate salt; and, (b)said step of forming an agent capable of treating lactate salt to formthe second lactate salt includes isolating tertiary amine.
 20. A processaccording to claim 18 wherein:(a) said step of treating the fermentationbroth comprises treating the fermentation broth with an alkaline earthmetal carbonate; and, (b) said step of treating an aqueous solution oflactate salt with an agent to form a first salt comprises treating theaqueous solution with an agent to form an alkaline earth metalcarbonate.
 21. A process according to claim 18 wherein:(a) said step ofrecovering lactic acid from the second lactate salt includes a step ofvaporizing lactic acid from the resulting water-immiscible liquid phase,after said step of separating the non-aqueous phase from the aqueousphase.
 22. A process according to claim 21 wherein:(a) said step ofvaporizing comprises vaporizing the lactic acid under vacuum at anabsolute pressure of 0.2 to 100 mm Hg and a temperature of 80° C. to140° C.